1. Field of the Invention
The present invention relates to a data transfer method and device in a data processing system using a Direct Memory Access Mode (referred to herein as "DMA").
2. Description of the Prior Art
As shown in FIG. 1, a computer system is known which comprises a processor 1, a main memory 2, an interface circuit 3, an input/output controller 4 and an input/output device 5 with data transfer between these elements being effected through a data bus 6. The DMA system, in which data transfer is performed directly between the main memory 2 and the input/output device 5, has been conceived as the input/output control system for such computer system, in addition to a program control system resorting to the program of the processor 1.
Such a system is described, for example, in "Microcomputer Handbook Series: Digital Memories and Peripherals", PP2-193-PP2-215 (published by Digital Equipment Corporation).
According to that DMA system, block transfer of data is performed to the area of continuous addresses of the main memory 2, such as a core memory, so that the data transfer can be effected at high speed. Prior to execution of the DMA mode, therefore, the first address of the area of transferred data and the amount of data to be transferred from the main memory 2 must be fed to the interface circuit 3 by the program control of the processor.
Here, in case an image displaying device for displaying various images is used as the input/output device, the data displayed in the displaying device is seldom obtained from one area of continuous addresses in the main memory 2, but is generally obtained from a plurality of different areas of continuous addresses, which areas are scattered in the main memory 2.
In case the data thus scattered in the main memory is to be transferred, the prior art has a drawback in that, since, prior to initiation of the DMA mode of each address area, the first address of that address area and the amount of data to be transferred must be fed to the interface circuit at a low speed by the program control, and the time required for transferring the first address and the amount of the transferred data becomes greater as the number of address areas scattered in the main memory is increased, with a result that the time for displaying the data is accordingly elongated. On the other hand, even in the case where the data is displayed within a short time, there arises another drawback in that the amount of data to be displayed is accordingly restricted.